introduce notes-cache interface

Notes provide a fast lookup mechanism for data keyed by
sha1. This is ideal for caching certain operations, like
textconv filters.

This patch builds some infrastructure to make it simpler to
use notes trees as caches. In particular, caches:

  1. don't have arbitrary commit messages. They store a
     cache validity string in the commit, and clear the tree
     when the cache validity string changes.

  2. don't keep any commit history. The accumulated history
     of a a cache is just useless cruft.

  3. use a looser form of locking for ref updates. If two
     processes try to write to the cache simultaneously, it
     is OK if one overwrites the other, losing some changes.
     It's just a cache, so we will just end up with an extra
     miss.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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